Fluorine-containing alkenes with the terminal —CF═CH2 group are useful compounds as various functional materials, solvents, refrigerants, and blowing agents. Fluorine-containing alkenes are also used as the monomers of functional polymers, or starting materials of such monomers, for example, monomers for modifying an ethylene-tetrafluoroethylene copolymer. Further, the compound of the chemical formula CF3CF═CH2 (HFO-1234yf) has recently gained attention as it offers promising prospects as a refrigerant compound of low global warming potential.
As a process for producing fluorine-containing alkenes with the terminal —CF═CH2 group, a method is known in which the starting material chlorine-containing alkane or chlorine-containing alkene with the same number of carbon atoms as that of the target fluorine-containing alkene is converted into a fluorine-containing alkene by reaction with a fluorinating agent using a catalyst. For example, many methods have been reported in which a starting material compound is reacted with anhydrous hydrogen fluoride used as a fluorinating agent in a gas phase in the presence of a fluorination catalyst that includes chromium oxide or fluorinated chromium oxide.
However, as a rule, the methods using chromium oxide or fluorinated chromium oxide inevitably produce certain amounts of plural by-products that cannot be converted into the target, causing problems such as reduction in the yield of the target fluorine-containing terminal alkene, and complications in the purification step. The latter is particularly problematic because the fluorine-containing alkene containing the same number of carbon atoms as the target, and having the terminal —CF═CHW group (W is F or Cl) has a boiling point close to that of the target. Generally, by-product production increases as the conversion of the starting material is increased by, for example, raising the reaction temperature.
Further, it is often the case in the foregoing methods that the starting material or intermediates that can be converted into the target remain at the reactor outlet. Though recycling of these starting material and intermediates in the repeated step of collecting the materials from the reactor outlet and feeding it back to the reactor inlet is possible for more efficient use, nonreusable by-products are concentrated during the production and hinder the producing process.
Methods are proposed in which the target is obtained by a fluorination reaction with anhydrous hydrogen fluoride in a gas phase or liquid phase in the presence of an antimony catalyst such as antimony chloride, followed by dehydrohalogenation (see, for example, Patent Literatures 1 and 2). While these methods are relatively effective at suppressing the production of by-product compounds, there is a problem of handling because the antimony chloride is sensitive to moisture and oxygen, and is easily deactivated. Further, the antimony catalyst cannot be used for extended time periods even in the absence of contact with moisture or oxygen, and a complicated reactivating process using a chlorine gas or the like is required.
Even with the reactivating process, difficulties remain in stably using the antimony catalyst for extended time periods, because antimony chloride or fluorinated antimony fluoride has a low boiling point and a low melting point, and the antimony catalyst flows out from the fixed layer of the catalyst when used in a gas phase. Further, because antimony chloride is highly corrosive to materials such as metal, use of expensive materials is necessary to prevent corrosion particularly in a liquid phase reaction.
The chromium oxide catalyst or fluorinated chromium oxide catalyst is less problematic than the antimony catalyst, because these catalysts are more stable and less corrosive, and are therefore easier to handle in industrial use. However, the conversion of the starting material is considerably poor in the reaction conditions described in, for example, Patent Literature 1 (for example, under the preferred temperature condition of about 30 to 200° C.), and the target fluorine-containing alkene cannot be obtained with high efficiency.
Patent Literature 3 describes a method intended to improve the conversion of the starting material by raising the reaction pressure in a system using a chromium oxide catalyst. However, the product produced by this method contains not only the target fluorine-containing alkene, but large amounts of fluorine-containing alkane that results from the addition of hydrogen fluoride to the fluorine-containing alkene. The fluorine-containing alkane can be thought as an intermediate of the target fluorine-containing alkene, and Patent Literature 3 describes reusing the intermediate or starting material by returning it to the reactor. However, these materials cannot be efficiently converted into the target fluorine-containing alkene, even when circulated in the reactor under the reaction conditions described in this publication.